At a low-pressure part of a nuclear power turbine, a geothermal turbine, or a thermal power turbine, a temperature and a pressure of steam being a working fluid are low. Accordingly, a part of the steam is condensed during expansion work to be water droplets to adhere to an inner wall of a steam passage, a stationary blade, and a rotor blade. The water droplets generated at the steam passage grow into the water droplets whose particle diameters are large. The water droplets whose particle diameters are large collide with a leading edge and so on of the rotor blade, and thereby, the rotor blade is eroded and a collision resistance relative to a rotation of the rotor blade is generated to lower turbine efficiency.
Here, a flow of the steam and so on in a vicinity of a turbine stage at a final stage in a general low-pressure turbine is described. FIG. 12 is a view illustrating a meridian cross section in a vicinity of a final turbine stage in a conventional low-pressure turbine. Note that in FIG. 12, a dotted line represents a streamline of the steam, and a solid line represents a trace of the generated water droplets.
As illustrated in FIG. 12, diaphragm outer rings 310a, 310b, diaphragm inner rings 311a, 311b are included at an inner side of a casing 300. Between the diaphragm outer rings 310a, 310b and the diaphragm inner rings 311a, 311b, plural stationary blades 312a, 312b are supported in a circumferential direction to make up a stationary blade cascade.
At an immediate downstream side of the stationary blade cascade, a rotor blade cascade in which plural rotor blades 322a, 322b are implanted into rotor disks 320a, 320b of a turbine rotor in the circumferential direction is made up. A single stage turbine stage is made up by the stationary blade cascade and the rotor blade cascade positioning at the immediate downstream thereof. In FIG. 12, a final turbine stage 330 and a turbine stage 331 at upstream side for one stage than the final turbine stage 330 are illustrated. At the rotor blades 322a, 322b, a speed energy of the steam expanded at the stationary blades 312a, 312b is converted into a rotational energy to generate a motive power.
As illustrated in FIG. 12, the steam expands along the steam passage which increases in size as it goes downstream. The water droplets are generated at a part where the pressure and the temperature of the steam descend, for example, at the turbine stage 331 which is at upstream for one stage than the final turbine stage 330.
A part of the generated water droplets flows toward the diaphragm outer ring 310a of the turbine stage 330 affected by the centrifugal force and the coriolis force. Accordingly, a lot of water droplets adhere to an inner surface of the diaphragm outer ring 310a of the turbine stage 330 to form a water film.
Besides, remaining water droplets collide with and adhere to a surface of the stationary blade 312a to form the water film. A water film reaching a trailing edge of the stationary blade 312a is blown and torn off by a steam flow at the trailing edge to be the water droplets. The water droplets collide with the rotor blade 322a at the immediate downstream side to erode the rotor blade 322a, activate a force in a reverse direction with a rotational direction to lower turbine efficiency.
Here, FIG. 13 is a view illustrating a distribution of wetness of the steam in a blade height direction of the stationary blade at the final turbine stage of the conventional low-pressure turbine. Note that a vertical axis represents a blade height ratio in which each blade height position is divided by a blade height. For example, when the blade height ratio is “1”, it indicates a blade tip of the stationary blade, and when the blade height ratio is “0” (zero), it indicates a blade root of the stationary blade. As illustrated in FIG. 13, the wetness becomes high at the tip side of the stationary blade, namely, at the diaphragm outer ring side. Accordingly, an adverse effect caused by the generated water droplets becomes remarkable at the diaphragm outer ring side.
In the conventional steam turbine, a technology to remove the generated water droplets and water film has been studied to suppress the erosion by the water droplets and the lowering of the turbine efficiency. As the technology to remove the water droplets and the water film, there is a technology providing plural through holes in the circumferential direction of the diaphragm outer ring to remove the water film adhered to an inner surface of the diaphragm outer ring.
However, when the plural through holes are provided in the circumferential direction of the diaphragm outer ring, the through holes are formed at a limited area between the stationary blade and the rotor blade, and therefore, it is impossible to enough expand a bore diameter. Accordingly, it is necessary to form a lot of through holes in the circumferential direction to uniformly remove the water film and the water droplets in the circumferential direction. This incurs complication of a manufacturing process, and increase in manufacturing cost.